A large number of electrophotographic methods have been conventionally known. Known electrophotographic methods generally involve: utilizing a photoconductive substance first to form an electrostatic latent image on an image bearing member (photosensitive member) by various means; next, supplying the latent image with toner to provide a visible image; obtaining a toner image; transferring the toner image onto a transfer material such as paper as required; after which the toner image is fixed to the transfer material by using heat pressure to provide a copied article.
Of those development modes, a one-component development mode is preferably used because a developing unit to be used in the mode is of a simple structure, causes a small number of troubles, and can be easily maintained. The one-component development mode involves the use of a one-component developer (which may hereinafter be referred to as “toner”). The mode involves: applying charge to toner particles by means of friction between a layer thickness regulating member (which may hereinafter be referred to as “blade”) and the developer and friction between a developer carrier (which may hereinafter be referred to as “developing roller”) and the developer; applying a thin layer of the developer onto the developing roller; conveying the developer to a developing region where the developing roller and an electrostatic latent image bearing member are opposed to each other; and developing an electrostatic latent image on the electrostatic latent image bearing member to visualize the image as a toner image.
The method enables the toner to be sufficiently subjected to triboelectric charging by the formation of a thin layer of the toner, but needs the uniform application of the developer onto the developing roller before development in order that the electrostatic latent image may be faithfully reproduced, and the resolution and definition of an image may be improved. However, in association with a recent increase in print speed, a strong mechanical stress is apt to be applied to, for example, a portion where the developing roller and the blade are close to each other, and a regulating force exerted by the blade on the developer on the developing roller becomes uneven, with the result that it is difficult to form a uniform thin layer of the toner. In addition, a shear force to be applied to the developer in a developing unit increases, thereby causing the deterioration of the developer, reductions in image quality and density, and a fogging phenomenon. Further, when images each having a high printing ratio are continuously developed, a reduction in density occurs in a stripe fashion owing to the insufficient supply of the toner to the developing roller.
In particular, in the case of a magnetic one-component development mode in which magnetism generating means is incorporated into a developing roller and magnetic toner obtained by incorporating magnetic particles into toner particles is used for preventing toner scattering, it is difficult to apply a developer uniformly to the developing roller owing to a magnetic binding force on the developing roller and an increase in stress in association with an increase in specific gravity of each toner particle.
To alleviate those problems, a method involving adding a large amount of a fluidity imparting agent such as a silica fine particle to a developer and a method involving adding two kinds of materials, that is, silica and titanium oxide have been proposed (see Patent Document 1). However, none of those methods is sufficient to achieve compatibility between charging stability and resistance against a mechanical stress.
In addition, methods each involving adding a strontium titanate particle having a small particle diameter or a composite particle composed of strontium titanate and strontium carbonate to a toner particle have been proposed (see Patent Documents 2 and 3). Particles used in those methods each have an excellent abrasion effect because each of the particles has a fine particle diameter, and the content of coarse particles in the particles is small. The particles used in those methods are effective in preventing the filming or fusion of toner onto an electrostatic latent image bearing member. However, at the same time, the particles used in those methods impair the fluidity of the toner. Accordingly, in each of those methods, it has been difficult to form a uniform thin layer of a developer on a developing roller in a developing step.
As described above, in order that a high-resolution, high-definition image may be stably obtained over a long time period irrespective of an environment, toner having not only a stable charging ability but also strong resistance against a mechanical stress has been required.
Efforts have been conventionally made to cope with such problems on the basis of measures for toner. However, such efforts are still susceptible to improvement.
In addition, in recent years, a photosensitive member having a photoconductive layer containing amorphous silicon and a surface protective layer (which may hereinafter be referred to as “amorphous silicon photosensitive member”) has been often used for the purposes of pursuing improvements in durability and image quality, and achieving a maintenance-free photosensitive member. In particular, an amorphous silicon photosensitive member drum is excellent in wear resistance because its surface layer is hard. Accordingly, the drum is suitably used in a use environment where images are continuously printed at a high speed over a long time period.
A digital mode involving the use of, for example, a laser light scan or an LED array as a light source has become the mainstream of latent image exposing means for a photosensitive member in order to correspond to the need for print-on-demand (POD). In this case, an appropriate one is chosen from two kinds of methods: a reversal development mode involving writing an image portion as a latent image with, for example, laser and causing toner to adhere to the portion and a regular development mode involving writing a non-image portion as a latent image and causing toner to adhere to a portion except the portion. The reversal development mode is suitably employed from the viewpoints of the emission intensity, response speed, and lifetime of a light source.
On the other hand, in a transferring step or a cleaning step, upon separation (stripping) of toner electrostatically adsorbed to the surface of a photosensitive member which moves at a high speed, a phenomenon in which charge opposite in polarity to the charged polarity of the toner is passed to the surface of the photosensitive member, that is, an electrostatic discharge phenomenon occurs. This is a peeling discharge phenomenon which occurs between the photosensitive member and the separated toner.
A discharge amount itself in association with the peeling discharge is extremely small. However, when the particle diameter of the toner is small (μm order), discharge converges on an extremely small area where the toner is in direct contact with the photosensitive member, and the resistance of the toner itself is high, the discharge amount may eventually become energy capable of breaking a charge blocking ability near the surface layer of the photosensitive member.
The voltage resistance of an amorphous silicon photosensitive member is typically high in the polarity direction of the charge of the photosensitive member, but is extremely low in the opposite polarity direction. Accordingly, when peeling discharge occurs on a side opposite in polarity to the charged polarity of the photosensitive member, and continues for a long time period, the charge retaining performance of the surface layer of the photosensitive member at the portion is apt to be finely broken. The reversal development mode is characterized in that toner and a photosensitive member are identical in polarity of charge to each other as follows: the charged polarity of the toner is positive and the charged polarity of the photosensitive member is positive, or the charged polarity of the toner is negative and the charged polarity of the photosensitive member is negative. Therefore, the polarity of peeling discharge occurring upon separation of toner from the surface of a photosensitive member is opposite to the charged polarity of the photosensitive member. Accordingly, particularly when an amorphous silicon photosensitive member is used, the charge retaining ability of the surface layer of the photosensitive member is apt to be finely broken. As a result, potential unevenness on the surface of the photosensitive member, and image density unevenness in association with the unevenness are apt to occur. Further, the local occurrence of a high electric field causes a leak phenomenon to break the photosensitive member itself. As a result, there arises a problem in that a black dot (hereinafter, this phenomenon is referred to as “black spot”) occurs on an image to reduce the print quality of the image remarkably.
In addition, the frequency at which, or the extent to which, such peeling discharge occurs tends to increase with increasing speed at which toner is stripped from the surface of a photosensitive member (in other words, the circumferential speed of a photosensitive member drum=a process speed), increasing bearing amount of developed toner on the surface of the photosensitive member, or increasing charge amount of the toner. Accordingly, the peeling discharge has started to distinguish itself as a serious problem in a recent trend, that is, an increase in print speed.
Under such circumstances, for the purpose of avoiding a peeling discharge phenomenon on the surface of an amorphous silicon photosensitive member, a method of controlling the resistivity of the surface layer of the photosensitive member to a low value (see Patent Document 4), and a method of controlling a relationship between the thickness and resistivity of the surface protective layer of the amorphous silicon photosensitive member to fall within a specific range (see Patent Document 5) have been proposed. In addition, a method involving constituting the structure of the amorphous silicon photosensitive member in an arbitrary manner to avoid the dielectric breakdown of the photosensitive member resulting from peeling discharge (see Patent Document 6) has been proposed.
On the other hand, a method involving adding a specific compound to toner to avoid a peeling discharge phenomenon on the surface of a photosensitive member (see Patent Document 7) has been proposed.
The methods proposed in Patent Documents 4 to 7 are each an effective method in terms of the suppression of a peeling discharge phenomenon or leak phenomenon on the surface of a photosensitive member. At present, however, in consideration of product design with an additionally high degree of freedom, an additional increase in number of alternatives has been demanded of those means for achieving the avoidance of a discharge phenomenon.
In addition, cleaning involving the use of a cleaning member has been performed for removing transfer residual toner from an image-bearing member in many cases. A mode in which a blade-like elastic member is brought into press contact with an image bearing member to sweep transfer residual toner has been often employed because the elastic member is of a simple structure. However, such blade may cause the following phenomenon: the reversal (turn) or chatter of the blade occurs, or the tip of the blade chips owing to friction between the image bearing member and the blade in long-term use, so a developer evades.
In addition, an inconvenience is apt to occur at a portion where a member except an image bearing member and the image bearing member are in contact with each other even in a constitution free of any cleaning step. For example, when contact charging is employed, an image bearing member may be nonuniformly charged owing to the contamination of charging means. In addition, contact developing means is used, a developer may be insufficiently charged owing to the fusion of the developer to, for example, a developing roller. Further, when contact transfer is performed, a transfer void due to the generation of a flaw on transferring means occurs in some cases.
Patent Documents 8 to 10 each propose a reduction in frictional force by such roughening of the surface of an image bearing member that an area of contact between a member contacting with the image bearing member and the surface of the image bearing member reduces with a view to solving those detrimental effects occurring between the image bearing member and the member contacting with the image bearing member.
However, each of the proposals still involves problems such as the difficulty with which such roughened surface is produced and a large influence on image quality.
In addition, those surface-roughening treatments each involve the following problem: a larger amount of irregularities than necessary are present on the surface of a photosensitive member, a fine particulate liberated product of a developer or a material of which the developer is constituted, in particular, a fluidity imparting agent or the like accumulates particularly at a recessed portion in the surface, and the developer is apt to fuse with the surface of the photosensitive member owing to the accumulation to cause a detrimental effect on an image.
In recent years, the following proposal has been made: a surface layer having high hardness is provided on an image bearing member so that the amount in which the member is shaved is reduced, and the lifetime of the member is lengthened (see Patent Document 10). However, as a result of an increase in hardness of the surface layer of the image bearing member, friction between the image bearing member and a member contacting with the image bearing member tends to increase to accelerate the above-mentioned phenomenon.
Various proposals have been made also for a developer. For example, Patent Document 1 described above proposes a method involving adding two kinds of materials, that is, silica and titanium oxide. In the method, silica and titanium oxide fine particles are apt to accumulate at a recessed portion in a photosensitive member subjected to a surface-roughening treatment, so an image bearing member is apt to be flawed, and the fusion of a developer is apt to be caused.
In addition, Patent Documents 2 and 3 described above each propose a method involving adding a strontium titanate particle having a small particle diameter or a composite particle composed of strontium titanate and strontium carbonate to a toner particle. In an image bearing member the surface of which is subjected to shape adjustment and to roughening, it has been difficult to remove a product liberated from a developer accumulating at a recessed portion even by using each of those additives.
As described above, not only an improvement in each of an image bearing member and a developer but also an improvement in performance based on a combination of the image bearing member and the developer has been needed for obtaining a good image stably while suppressing damage to an electrophotographic constituent member over a long time period.    Patent Document 1: JP 2002-372800 A    Patent Document 2: JP 10-10770 A    Patent Document 3: JP 2003-15349 A    Patent Document 4: JP 2002-287390 A    Patent Document 5: JP 2002-357912 A    Patent Document 6: JP 2002-287391 A    Patent Document 7: JP 2005-128382 A    Patent Document 8: JP 53-92133 A    Patent Document 9: JP 52-26226 A    Patent Document 10: JP 57-94772 A